Four wire circuits



y 1933- L. ESPENSCHIED 1,917,960

FOUR-WIRE CIRCUITS Filed April 24, 1931 2 Sheets-Sheet 1 Central Ofji'ce Sascriberls Station I C. 0. .Modula/t s Subscrzlhen's Station, l 1 I l 8 l 8 l g HDC X2 2 M11. 12am. 2 am. Jaw.

Central Office 3 Z'L'Zt. Statwlw E}? Dem. L

l I i Sta.

INVENTOR I lEspensc/u'ed ATTORNEY VAT.

Patented July 11, 1933 UNITED STATES i=ri earn; a"

PHONE AND TELEGRAPH COMPANY, A CORPQRA'IIUN OF NEW' YORK 'FOUR VJIRE CIRCUITS Application filed April 24,

This invention relates to transmission systems, and more particularly, to arrangements for obtaining the equivalent of fourwire operation in the local subscribers circuits of a telephone system.

The type of local telephone circuit in general use today, employs one and the same transmission path for both sending and receiving. On the other hand, the present tendency in long distance telephone circuits is to employ separate transmission paths for transmission in opposite directions. fihesc separate paths for opposite directional transmission may be physically distinct wire circuits as in the case of'four-wire voice frequency transmission, or they may be paths. of different frequencies derived from a single physical circuit as is done in carrier transmission. By analogy to the true four-wire method, the latter type of transmission has sometimesbeen called equivalent four-wire operation.

The reasons for employing two separate paths for opposite directional transmission in long distance circuits,

are principally the desire to avoid the'necessity for introducing balance at repeater points (such balance be ing diflicult to secure and maintain, and being objectionable from' the standpoint of echoes), and the desire to avoid"near-end crosstalk effects between the line circuits. The desire to employ four-wire operation in the long distance plant makes it desirable also to have four-wire operation in the local circuits when these are used for long distance connections lVith the present prac-v ties, the fact that the local circuits are twowire, imposes a serious handicap upon long distance foui wil'e operation.

Accordingly, it is proposed by the invention to obtain the equivalent of four-wire op eration in tie local plant including the subscribers line, by the use. of carrier frequencies for transmission in one or both. directions over the local subscribers circuits. In'the preferred and most simple form of the invention, carrier transmission is used f in one direction only, thus enabling the modulatlng, demodulatmg and carrier supply apparatus to be confined to thecentral office,

1931. Serial No. 532,585.

and permitting the subscribers lines and sub-station circuits to be used in their present form.

The invention will now be more fully understood from the following description when read in connection with the accompanying drawings, in which Figure 1 shows a circuit arrangement embodying a formof the invcntionin which carrier trausmi is employed in the local plantfor transmission in both directions; Fig. 2

in the transmission circuit of Fig. 1,; Fig.3

is a diagram of a system employing the p in ciples of the invention in which carrier transmission is employed in the in one direction, site direction being at normal voice frequencies; Figs. 3a and local plant tion circuit are eliminated; Fig. l is a gram of a circuit arrangement embodying a form of the invention in which carrier trans:-

mission is employed for one direction, and

transmission in the opposite direction takes freouenc 'level and receives at another frol I] Q o o All subscribers in the ex qucncy level. change however, use the same carrier frequency fortransmitting. All. of the sub scribers also receive at a common carrier frequency, but the carrier frequency employed 'or reception is different from that used for transmitting. As shown inldigrl, the sub scribers line terminates at the central office in the jacl; in the usual manner, but at the ssion is a diagram-j SlIOWID the free UODCViiEUlSlZIilOIl involved transmission in the oppoare sub station cir cult arrangements by, which side tone effects 1n .the receiver of the subscribers sub-sta directions of transmis disclosed subscribers premises the two-wire line is branched into two two-wire circuits, one including a transmitter T, a modulator SH and a filter SF and the other including a filter S11, a demodulator SD and a receiver R. The modulator is supplied with a carrier frequency f and the detector translates the received signal band from the receiving carrier f The filter SF, is, oil course, arranged to transmit the carrier frequency f and l associated side bands, and the receiving liltor SF is arranged to pass the carrier irequency f and its associated side bands.

At the central oliice a cord circuit which terminates in two-wire plugs of the usual.

. type, is prorideiil with twobranches, the

.cy and its one-way devices A west to east branch includes a filter OF} passing the carrier frequency f and its associated side hands, a modulator OM and a'filt er OF: for passing the carrier frequenassociated. side bands. The cast to west branch similarly contains a filter OR for passing a carrier frequency f and its associated side bands, a modulator OM and a filte OF; tor passing the carrier trequency'j" a d its associated side bands.

T ie modulators OM and OM at the central otlice are supplied with a carrier frequency equal to the diilierence between the carrier frequencies f, and f this carrier tre quency being supplied by a source 3:. In order to prevent cross talk in the adjacent cords supplied from the same carrier soiu'ce, and A are included in the leads from the generator G to the two to prevent J.

A and A modulators ot the cord circuit. The carrier frequency to be employed in the transmittcr branch at the subscribefis circuit is supplied by means oi a source G at the central oiiice, and leads trom'this source are connested to the plugs of the cord circuit to energize tie modulators such as SM and SM at the subscriber-s stations. In order the connections from the generator (it, to the cord circuit from short-circuiting the 'l .=ur-r-:ire operation ot the cord-circuit. and in order also to prevent cross talk l o Li, rln 1-3-1 "rm f Q m in t u, oanei on its connei at Lam up Nan carrier trcqueucy source, the leads from the source (7. may include one-way devices such source G. No carrier frequency sourccneed to the normal voice level.

carrier component f be provided at the subscribers station. The resultant frequency translations taking place during transmission in the two directions will be as shown schematically in the diagram of Fi 2, in whichthe stepping up and down ot the frequencies in the frequeney spectrum as transmission passes through the different parts 01 the system, are indicated.

'The operation otthe circuit of Fig. 1 will be as follows: In Fig. l the carrier frequency 7, from the source G is applied through the amplifier A to the subscribers line and is transmitted through the filter SE to the output side of the modulator SM. The modulator SM may be of any well known type as, for example,'a vacuum tube modulator, so that when the telephone transmitter T is actuated its grid potential is varied, thereby varying the impedance of the plate circuit of the modulator to the carrier frequency f Consequently, the carrier frequency is modulated and passes over the subscribers line, through the filter OF 1 in the cord circuit, to the modulator OM. Here the modulated carrier current ott'requency 7' beats with the locally supplied lated carrier 7', is stepped down to the modulated carrier frequency f which passes through the filter OF in the cord circuit, to the subscribers line at the right of the diagram. At the subscribers station modulated carrier frequency 7'': passes through the filter SF and enters the detector SD which detects therefrom and transmits to the re ceiver R, the original signal generated at the transmitter T. I

lransmission in the opposite direction is similar to that already described. v The same carrier trequency f is transmitted through the one-way device l to the subscribers line at the right and passes through the filter S1 to the modulator SM where it is modulatod by the voicev waves from the transmitter T. Modulated carrier frequency f now passes over the subscribers line and through the filter Oil of the cord circuit to themodulator Olv' The modulator steps the modulated carrier frequency f down to the modulated carrier frequency f which passes through the filter OF to the subscribers line at the left. At the subscribers station the modulated carrier frequency f passes through the filter SF: and is detected by the detector SD so that the signal-corresponding to that generated by the transmitter T is received in thereceiver R.

The arrangement of Fig. 1 involves the use of modulating and detecting apparatus, as well. as filters, at the subscribers station, which makes the sub-station apparatus much less simple than the present sub-station apparatus, and furthermore, addsto its cost. These ditficulties may be overcome 5} so that the moduhowever, if in the two-wire parts of the sys tem transmission takes place in. one direction only at carrier frequency, and in the opposite direction the voice currents are transmitted at ordinary voice frequencies. An arrangement embodying these principles is shown in Fig. 3. V

In this figure the subscribers sub-set is of the standard type involving the usual. transmitter T, receiver it and induction coil. The cord circuit at the central oilice however, in-* volves two branches as before, each branch including deniodlating apparatus as indicated at D and D. The carrier frequency to be utilized for transmission in one direction is generated by a source G and is applied to the two sides of the cord circuit through one-way devices A and A which serve the double purpose of preventing energy from being by-passed from one side of the cord circuit to the other, and of preventing cross tall: from one cord circuit into another. This latter feature is especially important as all. subscribers transmit at the same carrier frequency. Each side of thecordc rcuit is provided with filters, at highpass filter HP sing arranged on the inputside of the demodulator D, and a low-pass iilter LP being arranged on the output side of said demodulator. Similarly, in the o, posite directional path of the cord circuit, filters HP and LP are arranged on the input and output sides respectively, of the demodulator D.

In this systemno special modulator is provided, but the subscribefis telephone transmitter is used as the modulator. The carrier frequency from the source G passes through the oiie-way device and through the high-pass filter HP over the subscribers line to the-subscriber's station at the left.

he carrier frequency passes through both the subscribers transmitter T and the subscribers receix 'er R, but does not produce an. auoible sound in the latter because of its high frequency. \Vhen the subscriber spea is into the transmitter T however, the variable esistance of the transmitter causesvariations in the amplitude of the carrier fre quency flowing over the subscribers line, with the result that the modulated carrier frequency passes-through the filter HP into the demodulator D at the cord circuit. The

. demodulator D .detectsthe signal corresponding to the transmitter T, and the detected voice currents pass through the low-pass filter LP to the subscribers line at the right, the carrier frequency in the output of the demodulator being suppressed by the filter LP. At the subscribers station at the right the voice currents at ordinary voice frequency pass into the telephone receiver in the usual manner. 1

In the case of transmission in the opposite direction, the same carrier frequency from the source G passes through th'ehigh-pass filter HP to the subscribers line. at the.

right. The suhscribers transmitter at the sub-stationmodulates the carrier frequency which passes through the filter HP to the demodulator D. The demodulator D de tects the original Voice band which passes through the low-pass filter LP over the subscribers line to the receiver It of the sub-- scrihers station at the left. I

It will be noted, of course, thatthe subscribers transmitters in this systeinare not energizcd by direct current, but are energized by the carrier frequency. from the central oilice source G, the carrier frequency beingsupplied to theyariouscord circuits in a manner somewhat analogous to thecoinmon battery arrangement now commonly used in telephone systems. lVith the system oi Fi there may be also some demodulationof the modulated carrier frequency resulting from the action of the transmitter T in thelocal receiver'R which may act as a.

very ineilic'ient demodulator. This would, of course, result in side tone in the receiver.

here it isdesired to eliminate this side any well known type, such as shown Within the audible range, the carrier frequency currents, both modulated and unmodulated, may be heard in the receiver. An arrangement which may be employed to eliminate this effect, as well as side tone due to demodulation in the receiver, is shown in Fig. 36. Here a low-pass filter LP is included in the receiver circuit to suppress the carrier frequency, both modulated and uninodulated. i

In the arrangement of Fig. ,3, sep arate demodulators were used in the two sides of the cord circuit for transmission in the two directions. Fig. {l' shows. an arrangement in which the same demodulatoriD is employed for transmission in both directions. The' demodulator D is connected in a bridge path between the two sides of the cord circuit,

and the carrier frequency from the source G 1 is supplied-through a one-way ClEVlCQ A to the input side of the demodulator. The a'rrangementof the filters is also somewhat different, as twohigh-pass filters are inchided in the upper branch of the cord circuit on either side of the demodulator connection, and two lovwpass filters are included in the lower branch of the cord circuit on either side of the demodulator connection.

The subscribers sub-stations 'may be of standard type.

In operation, the carrier frequency from the source G is supplied to. the subscribers stations through the high-pass filters HP and HP, respectively. If the subscriber at the left speaks into his transmitter T the transmitter modulates the carrier frequency which passes through the high-pass filter HP of the cord circuit into the demodulator The demodulator D detects the original voice band from the modulated carrier, and

them to the subscribers lines through the low-pass filters LP and LP, as before.

It will be noted that in the arrangements above described, all of the sub-stations employ a common frequency band for, transmitting and a different common frequency band for receiving. This prevents a near end cross tall: from the transmitting band of one subscriber into the receiving band of another.

The arrangements so far described involve apparatus for establishing connections between local subscribers whose lines terminate atthe same exchange. For connection between. a subscribcrs line which. operates on the equivalent four-wire principle above described and a long distance circuit using an actually physical four-wire operation. any one of several arrangements may be employed. For example, connection may be made by means of a special type of cord circuit as illustrated in Fig. 5. Here the cord circuit terminates at the subscribe1"s end in a single plug P which may be connected to the subscribers jack J. The cord circuit is divided intoa transmitting branch and a receiving branch, the transmitting branch terminating in a plug TP which may be connected to the transmitting side of a physical four-wire circuit by inserting the plug into a jack .TJ. The receiving branch of the cord circuit similarly terminates in a plug RP which may be connected to the jack It, of'the receiving side of a physical four-wire circuit. The carrier frequency is supplied from a sourceG to the transmitting side of the cord circuit through an amplifier A. The transmitting side of the cord circuit also includes a demodulator D and a highpass filter HP. A low-pass filtrr LP is connected in the receiving branch of the cord circuit. It will be noted that the apparatus of this cord circuit involves just one-half of the apparatus of the cord circuit of Fig.

In operation, the carrier frequency from the source G is supplied through the filter HP to the subscribers line over the plug P and jack J. hen the subscriber talks into the transmitter 'l the carrier frequency is modulated and passes through the filter HP to the demodulator D of the cord circuit. The demodulator D detccts therefrom the voice band corresponding to the transmitter T, which is transmitted over the plug TP and jack TJ to. the transmitting line TL of the four-wire circuit. At a distant exchange the voice frequency band transmitted over the line TL will pass into the receiving side of a cord circuit similar to that illustrated in Fig. 5, and thence through a filter corresponding to LP, to. the distant subscriber station. In receiving from the distant station the transmis ion over the receiving line RL will be at voice frequencies whether the distant station is connected to an exchange of the ordinary battery type, or an exchange involving cord circuits of the type shown in, Fig. 5. carrier frequency is modulated by the sub scribers transmitter and is then detected in the central oflice cord circuit before passing into the receiving line RL, Q The arrangement of F 5 involves the difliculty that the central office operator would have to use a different type of cord long distance connections circuit for making from that employed for ordinary intra-ollice connections. Fig. 6, however, shows how a cord circuit of the type shown in Fig. 3, may be employed for making both local and long distance connections. terminates in two-wire plugs P'and P at the two ends. The plug P maybe inserted in the jack of a calling subscribefls line J, and the plug P may be inserted into either the jack of a called subscribers line as in Fig. 3, or into the two-wire jack J to which the transmitting line TL and the receiving line ItL of a physical four-wire circuit are joined. The physical four-wire circuit includes a low-pass filter LP in its transmitting line, and a modulator M in its receiving line. This is necessary because of the two-wire connection between the physical four-wire circuit and the fourwire part of the cord circuit, this two-wire circuit necessitating a translation of the voice frequency to a carrier frequency for transmission in. one direction. The modulator M in the receiving line of the four-wire circuit will be supplied with the same carrier frequency as is supplied to the cord circuit. For this purpose the carrier source G is connected through a one-way device A to the receiving side of the physical four-wire circuit.

Here the cord circuit In the latter case, of course, a

. scriber talks into his transmitter T this carri'er frequency is modulated and. the modulat-ed carrier current passes through the high-pass filter HP to the demodulator D which detects therefrom the voice hand core responding to that'generated at the transmittext T. This low frequency Voice band now passes through the low-pass filter LRover the plug P and the jack J: to the transmit ting line TL, passing through filter LP to the distant terminal of the four-wire circuit. The voice hand thus transmitted can not be transmitted over the receiving line; RL, as the one-way modulator M prevents such action. A voice band transmitted from the terminal of the physical four-wire circuitcomes in overthe receiving line BL and modulates in the modulator the carrier frequency supplied from the source G through the one-way device A. The modulated carrier current now passes over the jack J and plug P to the lower side of the cord circuit, being prevented from entering the transmitting line by reasonv of the low-pass filter LP". HP in the lower side of the cord circuit passes the modulated carrier-frequency to the demodulator D which detects therefrom the voice band corresponding to that generated atthe distant station, and the voice band now passes through to the subscribers station, and is'received by the subscribens receiver R in the usual manner.

It will be obvious that the general printhe subscribers station over all of the lines in the same frequency range, and receiving at the subscribers stations over all of the lines in a different common frequency range to that used for transmitting.

2. In a system in which a plurality of subscribers lines are arranged to be interconnected'in pairs for two-way conversation through the same switching exchange, the method which consists in transmitting from the suhscrihers stat-ion over all of the lines in the same frequency range, receiviin the subscribers station over all of the lines in a different common frequency rangeto that used for transmitting, and translating The high-pass filter from the one frequency'range to the other at the central station.

3; In a system in which two subscribers lines may he interconnected for two-Way conversation through a central oflice, the method which consists in transmittingin one directionover the subscribers line from one subscribers station to the central office in one frequency range, translating at the central office to another frequency range, transmitting over the second line to the second suhscribers station in said second frequency range, transmitting in the opposite direction from the second subscrihens station over his line to the central ofiice in said first frequency range,'translating at the central office to said second frequency range, and transmitting over the first subscribers line from the central oflice in said second frequency range.

4. In system in which a plurality of subscribers" lines are arranged to be interconnected in pairs for two-way conversation through the same switching exchange, the method which consists in transmitting from the suhscriber s station over all of the lilies in the same carrier frequency range, and

receivingat the subscribersstation over all of the lines inthe voice frequency range.

.5. In a system in which a plurality of subscribers lines are arranged to be interconnected in pairs for two-way conversation through the same switching exchange, the method which consists in transmitting from the subscribers station over all ofthe lines 1n the same carrier frequency range, re-.

ceiving at the subscribers station over all of the lines in the voice frequency range, and translating from one of said, ranges to the other range at the central olhce.

6. In a system in which two subscribers lines may be interconnected for two-way conversation through a central oliice, the.

method which consists in transmitting in one direction over the subscribers line from one subscrihers station to the central ollice in a carrier frequency range,'translat1ng at the central station to the voice frequency range, transnnttingover the second line to the second subscribers station in said voice frequency range, transmitting 1n the opposite direction from said second subscrihers station over'his line to the central oliice in the same carrier frequency range as was employed by said first line, translating at the central. ofiice to the voice frequency range, and transmitting over said first suhscrioers line from the central ollice in the voice fre- 1 quency range.

7. In a transmission system, a central of lice, a plurality, of subscribers lines adapted to be 'interconnected in pairs for two-Way conversation through said central oflice, means to transmit-voice signals over said lines to the central ofiice in the same fre-l office to translate the two channels of a twoway conversation over any pair of connected lines from one frequency range to the other.-

9. In a transmission system, a central office, apair of subscribers lines adapted to be connected for two-way conversation through said central office, means to transmit over the subscribers line from one subscribers station to the central office in one frequency range, means to translate to another frequency range at the'central office, means to transmit over the second line from the central office to the second subscribers station in said second frequency range, means to transmit in the opposite direction from the second subscriber-s station over his line to the central office in the first frequency range, means to translate at the central oflice to said second frequency range, and means to transmit over said first subscribers line from the central office in said second frequency range.

10. In a transi'nission system, a central office, a plurality of subscribers lines adapted to be iIltBlCOllHGCtQd in pairs for two-way conversation through said central office,

' means to transmit voice signals over. said lines to the central office in the same carrier frequency range, and means to receive over said lines in the voice range.

11. In a transmission system, a central plurality of subscribers lines adapted to be interconnected in pairs for two-way conversation through said central office, means to transmit voice signals over said lines to the central office in the same carrier frequency range, means to receive over said lines in the voice range, and means at the central office to translate from one of said ranges to the other.

12. In a transmission system, a central office, a pair, of subscribers lines adapted to be connected for two-way conversation through said central office, means to trans mit over the subscribers lines'from one subscriber's station to thecentral' office in a carrier frequency range, means at the centra l oflice to translate to the voice frequency range, means to transmit over the second line from the central office to the second subscribers'station in said voice frequency range, means to transmit in the opposite direction from the second subscribers station over his line to the central oflice in said carrier frequency range, means to translate said carrier range at the central office to the voice frequency range, and means to transmit over said first subscribers line from the to the waves so transmitted to the transmitting channel of the four-w1re circuit, means to receive Waves from the receiving channel of the four-wire circuit at said central oflice, and means to transmit waves corresponding to the waves so received over the subscribers line to the subscribers station in a frequency range different from said first frequency range.

14. I11 a'transmission system, a subscrib ers station, a central oflice, a subscribers line extending from the subscribefis station to the central oflice, a four-wire long distance circuit including a transmitting channel and a receiving channel associated with said central oflice, means to transmit from said subscribers station over the sub scribers line to the central oflice in one frequency range, means to apply Waves corresponding to the waves so transmitted to the transmitting channel of the four-Wire circuit, means to receive waves from the receiving channel of the four-wire circuit at said central office, means to transmit Waves corresponding to the waves so received over the subscribers line to the subscribers station in a frequency range different from said first frequency range, and means at the central station for translating from one frequency range to the other.

15. In a transmission system, a subscribers station, a central office, a subscribers line extending from the subscribers station to the central office, a four-wire long-distance circuit including a transmitting channel and a receiving channel associated With said central ofiice, means to transmit from said subscribers station over the subscribers line to the central ofiice in a carrier frequency range, means to apply waves corresponding to the Waves so transmitted to the transmitting channel of the four-Wire circuit, means to receive waves. from the receiving channel of the four-wire circuit at said central office, and means to transmit waves corresponding to the waves so received over the subscrlbers line to the subscribers station in the normal voice frequency range. a

16. In a transmission system, a subscrib ers station, a central office, a subscribers J line extending from the subscribers station circuit, meansrto receive waves from the re-.

ceiving channel of the four-wire circuit at said central oflice, means to transmit waves corresponding to the waves so received over the subscribers line to the subscribers station in the normal voice frequency range, and means at the central station for translaping from one frequency range to the ot ier.

. 17. In a transmission system, a central office, a plurality of subscribers stations,

lines extending from the subscribers stations to the central ofiice, link circuits at the central ofiice to connect the lines in pairs for two-way transmission, means associated wit two-way transmission, means associated with the link circuit to supply the same car rier frequency over two interconnected lines to the substations, means at each substation to modulate the carrier in accordance with speech, and means associated with the link circuit to translate the modulated carrier received fronreach line into normal voice frequency currents for transmission over the other line.

18. In a transmission system, a central ofiice, plurality of subscribers stations. lines extending from the subscribers stations to the central office, link circuits at the central office to connect the lines in pairs for two-way transmission, means associated with the link circuit to supply the same carrier frequency over two interconnected lines to the substations, means at each substation to modulate the carrier in accordance with speech, and means associated with the link circuit to translate the modulated carrier received from each line to a different common frequency range for transmission over the other line.

19. In a transmission system, a central office, a plurality of subscribers stations, lines extending from said subscribers stations to said central office, a four-wirelongdistance circuit including a transmitting channel and a receiving channel associated with said central office, link circuits at the central oflice to connect the lines in pairsfor two-way transmission or to connect any line to said four-wire circuit for long-distance two way transmission, means associated with said link circuit when interconnecting two subscribers lines to supply the same carrier frequency over the two interconnected lines to the subscribers stations, means at each substation to modulate the carrier in accordance with speech, means associated with the link circuit to translate the modulated carrier received from each line into waves of normal voice. frequency range for transmission over the other line, said link circuit having means to apply the waves in a translated frequency range to the transmitting channel of the four-wire circuit and to apply waves received from the recei ing channel of the four-wire line to the subscribers line in' the common frequency range employed for transmission from the central office over thesubscribers line to the substation.

20,111 a transmission system, a central office, a plurality of subscribers stations,

lines extending from said subscribers stations to said central office, a four-wire longdistance circuit includinga transmitting channel and a receiving channel associated with said central oflice', link circuits at the central office to connect the lines in pairs for two-way transmission or to connect any line to said four-wire circuit for long-distance two-way transmission, means associated with said link circuit when interconnecting two subscribers lines to supply the same carrier frequency over the two interconnected lines to the subscribers stations, means at each substation to modulate the carrierin accordance with speech, means associated with the link circuit to translate the modulated carrier received from each line into. waves of adifferent common frequency range for transmission over the other line, said link circuit having means to apply the waves in a translated frequency range to the transmitting channel of the four-wire circuit and to apply waves received from the receiving channel of the four-wire line to the subscribers line in the common frequency range employed for transmission from the central office over the subscribers line to the substation.

In testimony whereof, I have signed my name to this specification this 21st day of April 1931. v

LLOYD ESPENSCHIED. 

